1. Field of the Invention
The present invention relates to a thin film transistor for flat panel displays, and more particularly, to an organic thin film transistor that prevents the surface of an organic semiconductor layer from being damaged, and a flat panel display having the same.
2. Description of the Related Art
Studies on organic thin film transistors (OTFTS) for use in next generation display devices have been actively performed. OTFTs use an organic film instead of a silicon film as a semiconductor layer, and are classified into low molecule organic thin film transistors such as using oligothiophene and pentacene, and polymer organic thin film transistors such as polythiophene, according to the material that forms the organic film.
An organic electro-luminescence display device that uses an organic thin film transistor as a switching device includes at least two organic thin film transistors, for example, one switching organic thin film transistor and one driving organic thin film transistor, one capacitor, and an organic light emitting diode having an organic film layer interposed between upper and lower electrodes.
Typically, a flexible organic electro-luminescence display device uses a flexible substrate that includes a plastic substrate. The organic electro-luminescence display device that uses the plastic substrate has to be manufactured by a low temperature process since the plastic substrate has very low thermal stability.
Accordingly, an organic thin film transistor that uses an organic film as a semiconductor layer has been largely considered as an alternative for the switching device of the flexible organic electro-luminescence display device since the organic thin film transistor can be manufactured at low temperatures.
A pentacene thin film transistor that can reduce the thin film deposition time and improve the hole mobility has been disclosed in Korean Patent Laid-Open No. 2004-0028010. A device structure of an organic thin film transistor and a method of fabricating the organic thin film transistor that can improve the electrical performance of the transistor have been disclosed in Korean Patent Publication No. 2002-0084427. Also, a thin film transistor that can improve carrier mobility and on/off current ratio by incorporating a channel region into an organic compound having a radical has been disclosed in Japanese Laid -Open Patent No. 2003-92410.
An organic thin film transistor having a top gate structure includes a gate electrode formed on a substrate, a gate insulating film formed on the substrate, and source and drain electrodes formed on the gate insulating film, and a semiconductor layer formed on the source and drain electrodes and on the gate insulating film. In such an organic thin film transistor, the semiconductor layer is an unpatterned organic semiconductor layer formed on the entire surface of the substrate. As a result, there is a leakage current problem due to the accumulation of carriers, such as holes, between the semiconductor layer and the gate insulating film. To solve the above problem, the organic semiconductor layer can be patterned by laser ablation. However, laser ablation causes thermal deformation or recasting at edge portions of the patterned semiconductor layer. What is therefore needed is an improved structure and an improved method of making that overcomes the above problems so that there is no leakage current while having no thermal deformation or recasting.